This invention relates to apparatus for producing enveloped battery plates where the envelope comprises a porous separator having sealed edges along at least the bottom and sides of a battery plate.
Conventional plate-type secondary storage batteries typically comprise a plurality of positive and negative plates with interspersed porous, nonconductive separators. A plurality of plates are typically mounted within a battery cell containing an electrolyte solution. Recently the battery industry has been making inroads into the manufacture of batteries requiring little or no maintenance, specifically batteries which do not require the owners to add water to the batteries in order to maintain the level of electrolyte over the useful service life of the battery. While it is virtually impossible to totally prevent the loss of electrolyte during normal battery usage, it is desirable to minimize the rate at which the water in the electrolyte solution is depleted. Loss of electrolyte arises primarily through evaporation and gassing which occurs during charging of the batteries in which the water is subject to electrolysis producing hydrogen and oxygen gases which are then vented from the battery. The gases may also carry evaporated water vapor.
Typical lead acid batteries utilize lead-antimony alloys because of the effects of antimony on the physical characteristics of the lead material employed as battery grids. A disadvantage of antimonial lead alloys is that they generally result in increased gassing rates and the resultant increased loss of electrolyte solution. To overcome the disadvantages of the lead antimonial systems, the so-called low-or no-maintenance type batteries employ other lead alloys such as lead-calcium as grid materials. While the lead-calcium alloys reduce the rate of gassing and thereby reduce the rate of depletion of electrolyte, there are several disadvantages to the lead-calcium systems. For example, lead-calcium alloys are subject to creep or growth of the plates during usage. If the plates grow excessively, this may result in shorting of the battery by the growth of the plates or grid materials beyond the extent of the separator material whereby shorting may occur. Additionally, such grid materials are subject to "mossing" or "treeing"effects whereby dendrites of lead grow to form fine strands of conductive material which may extend around or through the separators and result in shorts between adjacent plates.
Conventional lead acid batteries typically include mud rests or upraised partitions in the bottoms of the batteries for supporting the plates and also for providing a space where particles of the battery paste which become dislodged from the plates can settle to the bottom of the battery. This precipitation of the paste material results in a higher concentration of lead sulfate in the bottom of the battery which promotes the mossing or treeing effects near the bottom of the battery. The mud rests were necessary however to prevent the precipitating material from falling into a position between adjacent battery plates which would increase the tendency to short the battery plates.
The battery industry has recognized the foregoing problems and attempted to provide enveloped battery plates which would retain the particles of lead paste within the confines of the enveloped separators while at the same time acting to reduce the tendency of the batteries to tree or moss as previously described. U.S. Pat. No. 2,934,585 issued to Zahn on Apr. 26, 1960 shows an enveloped battery plate which eliminates the need for mud rests in the bottom of the batteries. The adjacent separators forming an envelope are heat sealed along the edges to form an enclosed envelope. U.S. Pat. No. 3,013,100 issued to Mendelsohn et al. on Dec. 12, 1961 also shows an enveloped battery electrode where the separators are coated with a polyvinyl alcohol solution for forming a sealing surface between adjacent separator surfaces. U.S. Pat. No. 3,900,341 issued to Shoichiro et al. issued Aug. 19, 1975 also shows a folded separator, however, the edges of the separator are not completely sealed and allow space along the sides and bottom for escape of paste material. A sealant is applied to the edges of the separator to be sealed by heat sealing. U.S. Pat. No. 3,703,417 issued to Rosa et al. on Nov. 21, 1972 shows a plate envelope formed by two separators having edge sealant applied consisting of neophrene and polysulfone. The edges are heat sealed with an impulse sealer after the sealant is dried. U.S. Pat. No. 3,251,723 issued to McAlpine et al. on May 17, 1966 shows an apparatus for sealing pairs of separators along their edges to form envelopes around battery plates by a heat sealing method also.
The use of enveloped battery plates to permit elimination of the mud rests, enables a greater amount of electrolyte to be present over the tops of the battery plates. This prevents the plates from being exposed to the air until the electrolyte is depleted to a lower level than in conventional batteries thereby adding to the service life.
While the foregoing prior art patents recognize some of the desirable attributes of enveloped battery plates, there remain some problems with adapting their teachings to mass production techniques suitable for use in producing automotive batteries. Recently separator materials have become available which are made of heat sealable plastic such as polyethylene, polypropylene or vinyl such as polyvinyl chloride. While these separators may be heat sealed without the use of secondary adhesives or other material applications as suggested by some of the foregoing patents, they are subject to material degradation if the heat is improperly applied. Heat sealing may involve the use of external heat such as platens applied to the external surfaces of the separators to be sealed along the joint interfaces. This method of heating requires carefully controlled temperatures since the heat must be transferred all the way through the separator material in order to soften the separator sufficiently to result in bonding. This process is also relatively slow for mass production techniques.